The occurrence and the distribution of rhizobial populations naturally associated to Acacia seyal Del. were characterized in 42 soils from Senegal. The diversity of rhizobial genotypes, as characterized by polymerase chain reaction restriction fragment length polymorphism (RFLP) analysis of 16S-23S rDNA, performed on DNA extracted from 138 nodules resulted in 15 clusters. Results indicated the widespread occurrence of compatible rhizobia associated to A. seyal in various ecogeographic areas. However, the clustering of rhizobial populations based on intergenic spacer (IGS) RFLP profiles did not reflect their geographic origin. Four genera were discriminated on the basis of 16S rRNA gene sequences of the strains representative for the IGS-RFLP profiles. The majority of rhizobia associated to A. seyal were affiliated to Mesorhizobium and Sinorhizobium 64 and 29%, respectively, of the different IGS-RFLP profiles. Our results demonstrate the coexistence inside the nodule of plant-pathogenic non-N(2)-fixing Agrobacterium and Burkholderia strains, which induced the formation of ineffective nodules, with symbiotic rhizobia. Nodulation was recorded in saline soils and/or at low pH values or in alkaline soils, suggesting adaptability of natural rhizobial populations to major ecological environmental stress and their ability to establish symbiotic associations within these soil environments. These results contribute to the progressing research efforts to uncover the biodiversity of rhizobia and to improve nitrogen fixation in agroforestry systems in sub-Saharan Africa.

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Aims:  In order to understand the genetic diversity of Acacia tortilis ssp. raddiana‐rhizobia in Tunisia, isolates from nine geographical locations were obtained and analysed. Methods and Results:  Characterization using restriction fragment length polymorphism analysis (RFLP) of PCR‐amplified 16S rRNA gene and the intergenic spacer (IGS) between the 16S and 23S rRNA genes was undertaken. Symbiotic efficiency of the strains was also estimated. Analysis of the 16S rRNA by PCR‐RFLP showed that the isolates were phylogenetically related to Ensifer ssp., Rhizobium tropicii‐IIA, and Rhizobium tumefaciens species. Analysis of 16S‐23S spacer by PCR‐RFLP showed a high diversity of these rhizobia and revealed eleven additional groups, which indicates that these strains are genetically very diverse. Full 16S rRNA gene‐sequencing showed that the majority of strains form a new subdivion inside the genera Ensifer, with Ensifer meliloti being its nearest neighbour. Nodulation test performed on the plant host demonstrated differences in the infectivity among the strains. Conclusion:  Rhizobial populations that nodulate specifically and efficiently Acacia tortilis ssp. raddiana in representative soils of Tunisia is dominated by E. meliloti‐like genomospecies. Significance and Impact of the Study:  This paper provides the first clear characterization and symbiotic efficiency data of rhizobia strains nodulating A. tortilis in Tunisia.

plantations. Le nombre et la richesse des spores, de même que le PIM des sols étaient réduits dans les plantations d'arbres. Parallèlement, la quantité de plantes annuelles herbacées conservées hors des plantations d'arbres était plus grande que celle qui poussait sous les arbres plantés. La colonisation était plus forte dans le système radiculaire des plantes herbacées poussant dans la nature que dans celles poussant sous les arbres plantés. Les comparaisons nous permettent de conclure que le type de végétation modifie les communautés fongiques MA, et les résultats suggèrent d'adopter davantage des pratiques de gestion qui pourraient améliorer ou soutenir le développement de strates herbacées et favoriser ainsi les communautés fongiques MA.